Underwater sound transducers are devices that detect or generate sound in water to determine the location of objects in the water. The transducer converts electrical energy into acoustic energy or acoustic energy into electrical energy.
One type of transducer utilized by the prior art was a flextensional transducer. Flextensional transducers have wider bandwidths, lower operating frequencies and higher power handling capabilities than other types of transducers of comparable size. A flextensional transducer has a flexible outer shell or housing which is excited by one or more interior piezoelectric ceramic stacks. The piezoelectric stacks are driven in a length expander mode and are placed in compression between opposing interior walls of the shell. The elongation and contraction of the piezoelectric stacks impart a motion to the shell which, in general, radiates or couples energy into the water.
The piezoelectric properties of ceramic transducers vary with the stress experienced by the transducer's piezoelectric stack. Stress is supplied to the piezoelectric stack by the transducer's shell. During assembly of the transducer, a static, compressive prestress is applied to the piezoelectric stack. As the depth of the transducer increases, the transducer's shell experiences increased hydrostatic pressure which causes increased shell deflection. This results in a decrease in the amount of stress that is applied to the piezoelectric stack. Thus, the characteristics of the transducer are variable with depth and, in general, the maximum depth of operation of the piezoelectrically-driven flextensional transducer is governed by the amount of ceramic stress that may be removed from the piezoelectric stack without affecting its performance. For purposes of this discussion, the survival depth of the transducer is that ocean depth in which the piezoelectric stack fractures due to increased tensile stress.
Prior art flextensional transducers could operate at full power at some ocean depths; at reduced power at greater ocean depths (maximum operating depth) and at still greater ocean depths the transducer's piezoelectric stack would fracture and the transducer would not operate at all. Thus, if the flextensional transducer accidently descended beyond its survival depth and subsequently was raised to its maximum operating depth, the transducer would not function. In order to increase the survival depth of the transducer, the prior art would change the design of the transducer by increasing the thickness of the walls of the transducer and/or the size of the transducer. A disadvantage of the foregoing was that the modified transducer would resonate at a different frequency than the originally designed transducer. Thus, a trade-off had to be made between the survival depth of the transducer and the transducer's resonant frequency.